Nonwoven materials are made by the bonding of web like arrays of fibers or filaments. The materials can be made from staple fibers of discreet lengths by carding, wet laying, or the like, or they can be produced by laying or blowing filaments as they are melt extruded. The nonwoven materials made by these latter processes are commonly known as spunbonded or spunlaid and meltblown materials. More particularly, spunbonded materials are generally produced as continuous filaments to form fibrous materials such as fabrics, webbing, sheets, films, tapes, and the like. Meltblown materials are produced by a process in which extremely fine or super fine fibers of typically less than 10 microns in diameter are extruded under the influence of a dynamic flow of air and are collected on a screen or belt in the form of a nonwoven web or batt. As a result of the dynamic air flow, the fibers are drawn so that there is obtained a difference in birefringence, crystallinity and molecular orientation as compared to conventionally spun fibers.
Fibrous materials containing various polymeric fibers are, of course, well known in the prior art. Processes for preparing such fibrous materials from thermoplastic materials using a meltblown process have been described in publications such as Naval Research Laboratory Report (NRL) 30 No. 111437 of Apr. 15, 1954; NRL Report 5265 of Feb. 11, 1959, and Industrial and Engineering Chemistry, Vol. 48, No. 8 (1956), pages 1,342-1,346. Meltblown processes are also described in U.S. Pat. Nos. 2,374,540; 2,411,659; 2,411,660; 2,437,363 and 3,532,800. Methods for preparing spunbonded articles are described in British Pat. Nos. 1,055,187 and 1,215,537 and in U.S. Pat. Nos. 3,379,811 and 3,502,763.
U.S. Pat. No. 4,118,531 to Hauser, which is incorporated herein by references, discloses meltblown webs that comprise a mixture of microfibers and crimped bulking fibers which are used for thermal insulation. These webs are sold as Thinsulate.TM. by Minnesota Mining and Manufacturing Corporation, an insulation for clothing articles. However, the insulation is highly flammable and does not have the characteristic of reloft.
As is well known, meltblown materials have found utility in a broad range of applications. For example, it is known to use meltblown filaments, particularly those obtained from thermoplastic resins, in the preparation of battery separators, cable wrap, capacitor insulation paper, as wrapping materials, clothing liners, diaper liners, in the manufacture of bandages and sanitary napkins, and the like.
The problem with the prior art meltblown materials is that the large amount of thermoplastic materials utilized in the manufacture of the meltblown materials, Such as battings, render the battings highly flammable and particularly so because of the small diameter fibers, of less than 10 micrometers, that are utilized to provide an increase in surface area when compared to conventional fibers.
U.S. Pat. No. 4,837,076, by McCullough et al., which is herein incorporated by reference, discloses crimped, irreversibly heat set, carbonaceous fibers having a reversible deflection ratio of greater than 1.2:1 which can be used in preparing the fibrous materials of the invention.
U.S. Pat. No. 4,879,168 to McCullough et al. discloses an ignition resistant structure wherein conventionally spun thermoplastic fibers are blended with carbonaceous fibers. However, the thermoplastic fibers are of a relatively large diameter on the order of from 15 to 25 microns and, accordingly, are not as efficient for use as a thermal insulating material as compared to the meltblown fibrous materials of the invention. In the area of building insulation, the meltblown fibrous material of the invention is particularly effective as a thermal insulating material when compared to fiberglass.
It is understood that the term "fibrous material" as used herein refers to a multiplicity of randomly entangled fibers in the form or shape of a nonwoven sheet, fabric, web, batt, or the like, depending upon the loft and density of the material. The fibrous material can be in the form of a single ply or a multiplicity of superimposed or stacked plies.
The term "reloft" defines the ability of the fibrous material to return to its original dimension after the material has been subjected to a compression load of 15 psi for one hour at ambient temperature.
The term "microfiber" or "fibrils" used herein is well known in the fiber arts and is generally applicable to all polymeric fibers having an average diameter of less than 15 microns and, typically, less than 10 microns.
The term "crimp" generally defines the wariness or nonlinearity of a fiber expressed in the number of waves per unit length and and its amplitude. The wariness of the fiber can include different symmetrical or nonsymmetrical configurations such as sinusoidal, coil like, and the like.
The term "reversible deflection" or "working deflection" generally applies to helical or sinusoidal compression springs and is applicable to the crimped fibers employed in the present invention. Particular reference is made to the publication "Mechanical Design-Theory and Practice" MacMillan Pub Co., 1975, pp. 719 to 748: particularly Section 14-2, pages 721 to 724.
The carbonaceous fibers that are employed in the present invention are produced from polymeric precursor fibers such as, for example, oxidized polyacrylonitrile fibers, by heat treating the fibers in a nonoxidizing atmosphere to render the fibers carbonaceous. The term "carbonaceous fiber" is understood to mean that the carbon content of the fiber is greater than 65% but less than 98%, preferably less than 92% by weight, and that the carbon content has been increased as a result of an irreversible chemical reaction induced by heating the polymeric precusor fibers in a non oxidizing atmosphere. Fibers having a carbon content of greater than 98% by weight are known as graphitic fibers.
The term "permanent" or "irreversibly heat set" used herein applies to nonlinear carbonaceous fibers which possess a degree of resiliency and flexibility such that the carbonaceous fibers when stretched and placed under tension to a substantially linear shape, but without exceeding the tensile strength of the fibers, will revert substantially to their nonlinear shape once the tension on the fibers is released. The foregoing terms also imply that the fibers can be stretched and released over many cycles without breaking the fibers.
The term "Pseudoextensibility" or "Pseudoelongatability" refers to the elongatability of a fiber which results from the crimped or nonlinear configuration including any false twist that is imposed on the fiber.
The term "bending strain of the crimped fiber" as used herein is as defined in Physical Properties of Textile Fibers., W. E. Morton and J. W. S. Hearle, The Textile Institute, Manchester, 1975, pages 407-409. The percent bending strain resulting from the crimp on the fiber can be determined by the equation: EQU S=.sub.-- r/R.sub.-- .times.100
where S is the percent (%) bending strain, r is the fiber radius and R is the radius of curvature of bend. That is, if the neutral plane remains in the center of the fiber, the maximum percentage tensile strain, which will be positive on the outside and negative on the inside of the bend of the fiber, equals r/R.times.100 in a circular cross section of the fiber.
The term "stabilized" used herein applies to precursor fibers or fiber tows that have been oxidized at a temperature of typically less than 300.degree. C. For acrylic fibers prior to subjecting the fibers to a heat treatment to convert the precursor fibers to carbonaceous fibers. It will be understood that, in some instances, the fibers or fiber tow can also be oxidized by chemical oxidants at a lower temperature.